Stiles already has the legal right to drive in 1x01 - Wolf Moon. Based on California law, Stiles is 16 at the opening of the show, making the assumption that they are disregarding the restriction on driving with passengers.

Stiles has ADHD and takes Adderall to help him focus. He loves a mystery and, more, loves solving them, despite whatever trouble it may get him into. He is head-strong and often careless, at least in the beginning. It's clear that he and Scott have a history of mischievousness, of which Stiles is the instigator. However, as events in his life become threatening and serious, Stiles takes on the role of protector. His aptitude for problem-solving and desire to keep the people he loves safe force him to develop better and more thorough plans.